This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Prior SAXS results with control proteins of known structure and several gp140 trimer forms show that SAXS is a tractable method of analysis in our hands. All samples can be prepared on short notice for SAXS data collection and are currently in routine production to support other studies. Protein monodispersity is monitored by SEC with simultaneous static/dynamic light scattering as well as quantitative/qualitative Ab binding assays in a variety of formats. We expect to obtain overall shape and size information of different gp140-NAb complexes, generate models for the functional form of envelope spike if possible, and verify by cryo-EM structures of Env spikes and crystal structures of gp120 and gp41.